Typical automotive air conditioning systems use the familiar refrigerant cycle system of compressor, condenser, expansion valve and evaporator, through which pressurized refrigerant is continually pumped while the compressor is active. An inherent drawback of the typical system is that it is either on or off. That is, there is no thermal storage or reservoir capacity in such a system that can take over temporarily to maintain passenger comfort when the compressor is turned off for a short time, or which can be activated initially to provide cooling capacity when the system has been shut down for a relatively longer time.
A secondary loop system provides the potential for a reservoir of cooling capacity. In such a system, the conventional refrigerant loop is not used to directly cool the air that is blown into the passenger space. Instead, an intermediate, two fluid heat exchanger, typically known as a chiller, cools a secondary, non pressurized liquid by heat exchange with the cold refrigerant. The cooled secondary liquid is then pumped in a second circuit to one or more air to liquid heat exchangers, over which cooled air is blown into the passenger space. With the non pressurized liquid in the secondary loop, it is feasible to provide a large volume reservoir of "cold", from which cold liquid can continue to be pumped, even when the primary, refrigerant circuit and its compressor is shut down. Such a reservoir is not possible with just a refrigerant loop.
A potential drawback of the secondary loop system is the response of the reservoir after a long period of system shut down, or "soak" as it is sometimes called. Then, the large reservoir of "cold" becomes a large reservoir of "hot" instead, feeding the secondary loop with hot liquid until the whole system can be cooled down. Under those circumstances, the reservoir can actually lengthen the response time and delay passenger comfort.